The assignee of the present invention has pioneered a category of products now known as fixed disk expansion board products. The original version of the assignee's fixed disk expansion board product line is described in the assignee's U.S. Pat. No. 4,639,863, issued on Jan. 27, 1987, and entitled "Modular Unitary Disk File Subsystem" (hereinafter referred to as "the referenced '863 patent"). The disclosure of the referenced '863 patent is incorporated herein by reference. Reference is also made to other ones of the assignee's patents, including U.S. Pat. No. 4,647,997, issued on Mar. 3, 1987, and entitled "Aerodynamic Latch for Disk File Actuator" and U.S. Pat. No. 4,661,696, issued on Apr. 28, 1987, and entitled "Optical Encoder Which Uses a Rectangular Photodetector Array". Reference is further made to the assignee's following U.S. Patents and Applications: U.S. Pat. No. 4,712,146, issued on Dec. 8, 1987, and entitled "Thin and Compact Micro-Winchester Head and Disk Assembly"; Ser. No. 06/741,588, filed on June 4, 1985, for "Microprocessor Controlled Rigid Disk File Subsystem"; and U.S. Pat. No. 4,703,176, issued on Oct. 27, 1987, and entitled "Compact Polyphase Optical Position Encoder". These patents and applications describe and claim separate aspects of the fixed disk expansion board described in the referenced '863 patent, and the reader is referred to them for further details.
While the fixed disk expansion board described in the referenced '863 patent met with immediate and widespread commercial acceptance and success from its market introduction in mid-1985, the subsequent introduction of more complex software applications programs, such as spreadsheet and computer aided design, with increased on-line mass storage requirements, has created a hitherto unsolved need for greater data storage capacity in a form factor substantially no greater than that required by the disk drive subsystem 5 originally disclosed in the referenced '863 patent, namely a 31/2" diameter, dual fixed disk based subsystem meeting a width constraint not substantially in excess of one inch of overall thickness and which is suitable for plugging into and occupying a single electronics circuit board expansion slot location of a personal computer, such as the IBM.TM. Personal Computer, and equivalents thereof.
Past proposals for zoning disk data storage surfaces into zones of concentric tracks characterized by different data transfer rates to achieve a more uniform flux transition density over the useful radius of the storage disk, as exemplified by the Ottesen U.S. Pat. No. 4,016,603 for example, have required complex circuitry to accomodate the differing data transfer rate bandwidths associated with the zones. Other references relating to optimization of data transfer rate with disk radius which have also been considered in the preparation of this patent application describe a variety of variable data rate proposals and they include the Hasegawa U.S. Pat. No. 4,357,635; Stovall, Jr. U.S. Pat. No. 2,901,737; Hibner U.S. Pat. No. 3,524,172; Gleim et al. U.S. Pat. No. 3,375,507; Freeman et al. U.S. Pat. No. 4,314,287; and King et al. U.S. Pat. No. 2,843,841.
A hitherto unsolved need has arisen for a compact self contained fixed disk data storage subsystem which achieves considerably greater user data storage capacity without any concomitant increase in overall physical dimensions, including circuit complexity or size.